2. IMPRESSION
• A negative likeness or copy in reverse of the surface
or object
• negative replica of the hard and soft oral tissues
• Register or reproduce the form and relationship of
the teeth and oral tissues
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3. Ideal requirements of Impression
Materials
A pleasant odour, taste and acceptable color
Biocompatible ,non-toxic
Adequate shelf life for requirements of storage and
distribution
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Easy to use with minimum of equipment
Low enough viscosity to adapt to the oral
tissues,yet be viscous enough to be contained in
the impression tray
4. Should have adequate wettability of the oral
tissues
Set impression should show adequate elastic
recovery with no permanent deformation upon
removal from mouth
4
Dimensionally stable after setting over temperature
and humidity ranges
Compatible with cast and die materials
5. Readily disinfected without significant loss of
accuracy or loss of mechanical properties
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Must be cost effective
No release of gas or other by-products during
setting of the impression materials
8. CLASSIFICATON:
Classified on the basis of setting and elasticity.
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Mode of setting rigid Elastic
Set by chemical
reaction(irreversible)
Impression plaster, zinc
oxide eugenol.
Alginate, polysulfide,
polyether, silicone,
Set by temperature
change(reversible)
Compound, waxes Agar hydrocolloid.
9. Impression Plaster
• Used as mucostatic impression material for
edentulous patients
• Doesn’t compress and displace tissues during
seating of tray due to its fluidity
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• Applicable to patients with displaceable soft tissues
that should be recorded in a passive state
10. Composition
• ß-calcium sulphate hemihydrate
• Reacts with water to form calcium sulphate
dihydrate
• W/P ratio– 0.5-0.6
• Expansion and setting times controlled by
incorporating compounds designed to mediate
handling properties
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11. • Potassium sulphate added as an anti-setting
expansion agent
• Borax(retarder)- added to the powder to balance
the setting acceleration caused by Pot. Sulphate
and to bring the setting time under control
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• Alzarin red-to make clear distinction between the
impression and model
• Custom tray constructed using 1-1.5mm spacer
with acrylic resin or shellac
• Impression plaster can be used as wash material
• Techinque- “Puddling” the impression into place
12. • With remaining plaster in tray, the tray is seated in
single movement
• Then tray is gently moved from side-to side and antero-
posteriorly to take advantage of fluidity of material
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13. • Hemihydrate particles absorb moisture from the
surface of the oral tissues allowing intimate contact
between impression material and the tissues
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• Plaster impression material –very brittle and
fractures easily
• When undercut is involved, fracture the impression
to facilitate removal from mouth
• Fragments reconstructed to form completed
impression
14. • Beading of the impression done
• Coated with separating medium and cast in fresh
plaster
• Disinfection- achieved with a 10 min soak in sod
hypochlorite solution
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• Not used regularly due to mechanical limitations
• Used frequently as occlusal registration material
15. Zinc-oxide Eugenol Impression
Paste
• Composition-2 separate pastes dispensed in tubes
• One tube contains zinc oxide and vegetable or
mineral oil
• Other tube contains eugenol and rosin
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16. Composition
Components Percentage
Tube no 1 (base)
Zinc-oxide 87
Fixed vegetable or mineral oil 13
Tube no 2 (accelerator)
Oil of cloves or eugenol 12
Gum or polymerised rosin 50
Filler(silica type) 20
Lanolin 3
Resinous balsam 10
Accelerator solution(CaCl2) and color 5 16
17. Setting reaction of ZOE
• Ionic in nature
• Requires ionic medium in which it can proceed at
any desired rate
• 1st reaction-hydrolysis of zinc oxide to its hydroxide
form
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18. Zinc Oxide Eugenol
• When the 2 pastes are mixed,the phenol –OH of
the eugenol acts as a weak acid and undergoes an
acid-base reaction with zinc hydroxide
• Forms a salt- zinc eugenolate
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• Two further coordinate bonds are formed by
donation of pairs of electrons from methoxy oxygen
to zinc
19. • Disadvantage-
• Stinging or burning sensation caused by eugenol
• Orthoethoxybenzoic acid -substitute
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Manipulation
• Mixed on oil impervious paper or glass mixing slab
• Proper proportion of two pastes obtained by
squeezing 2 strips of paste of the same length,one
from each tube ,onto the mixing slab
20. • Flexible stainless steel spatula used for mixing
• 2 strips of contrasting colors combined with the
first stroke of the spatula ,mixing is continued for
approx 1 min, until a uniform color achieved
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21. • Shorten the setting time – by adding small amount
of Zinc acetate or additional accelerator or a drop
of water in the paste before mixing or by extending
mixing time
• Prolonging the setting time- cool spatula and
mixing slab
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• Paste of thick consistency –compresses the tissues
• Thin,fluid material results in little or no
compression
Advantage of heavier consistency – increased
strength
22. Dimensional stability
• Negligible shrinkage(less than 0.1%) may occur
during hardening
• No significant dimensional change
• Impression can be preserved indefinitely without
change in shape provided the tray material is
dimensionally stable
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Disinfection
• 2 % alkaline glutaraldehyde solution
• Immersed in solution for required time,rinsed and
poured immediately
23. Applications of ZOE
• Final impression of edentulous ridges
• As a wash impression with other impression
• As an interocclusal registration material
• As a temporary liner material for dentures
• As a surgical dressing
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24. Impression compound
• Also called “modelling plastic”
• Thermoplastic material
• Supplied in the form of cakes(red) and sticks
(green, gray or red)
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25. Composition
• Mixture of –waxes(principal ingredient)
thermoplastic resins
filler(increase viscosity and rigidity)
coloring agent
• Shellac, stearic acid and gutta-percha added to
improve plasticity and workability
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27. Type I (Lower fusing material)
• Cakes- as an impression
material for completely
edentulous patients, the
material is softened by
heat, inserted into the tray
and placed against the
tissues before it cools to a
rigid mass
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28. • Sticks- as a border
molding material for the
custom tray ,the material
is used before making the
final impression
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29. Type II( Higher fusing material)
• Used as an adaptation material which requires
more viscous properties
• Used for making primary impression of the soft
tissues and then used a tray to support a thin layer
of a second impression material such as ZnOE
paste, hydrocolloids or nonaqueous elastomers
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30. Manipulation
• Setting mechanism– reversible physical process
• Softening by heat
• Preheated and used warm (~450C)
• Then cooled to the intraoral temperature(370C) at
which it is fairly rigid
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• Once the impression tray is seated , it should be
held gently(passively) in position until the
impression cools below the fusion temperature
31. • Dimensional stability- allow thorough cooling of the
impression before removal from the mouth and to
construct the cast or die as soon as possible after the
impression has been obtained(at least within the
hour)
• Disinfection – 2% alkaline glutaraldehyde solution
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32. Thermal Properties
a)Thermal conductivity-low thermal conductivity
Significance
• During softening of the material, the outside will
soften first and inside last, so to ensure uniform
softening, the material should be kept immersed for a
long time in water bath
• The layer adjacent to tissues will remain soft . Thus it
is important to cool the compound thoroughly before
removing the impression
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33. b)Coefficient of Thermal Expansion- high (0.3%
acceptable)
c)Glass Transition Temperature
• The temperature at which the material loses its
hardness or brittleness on heating or forms a rigid
mass upon cooling
• Approx 39°C
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d)Fusion temperature– corresponds to a definite
reduction in plasticity of the material during cooling
• Above this temperature, the material remains
plastic while the impression is being made
• Approx 43.5°C
34. Significance of Fusion temp and
Glass transition temp
• Above Fusion temp,the fatty acids are liquid and
lubricate the softened material to form a smooth
plastic mass while the impression is being obtained.
• Thus all impressions with compound should be
made above this temperature
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• Once the impression tray is seated,it should be held
firmly in position until first fusion temperature and
later the glass transition temperature is reached
• Thus, impression is made above the fusion
temperature and removed after it cools down to its
glass transition temperature
35. Considerations for impression
compound
• Low thermal conductivity- adequate time needed
to attain thorough heating and cooling
• Incorporation of water(wet kneading)- excessive
flow of the material at mouth temp. producing
distortion as the impression is removed from
mouth
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36. • Tray used for impression must be strong and rigid
enough to support the material and to avoid
distortion of the impression
• Relatively high viscosity limits its ability to record
fine details
• Cast should be poured as soon as possible to
minimise distortion due to relaxation of the
compound
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• Safely disinfected by immersion in sod
hypochlorite, iodophors or phenolic
glutaraldehydes
